System and method for converting optical diameters of aerosol particles to mobility and aerodynamic diameters

ABSTRACT

A system and a method of measuring a particle&#39;s size in a select aerosol using the optical diameter of the particle to perform a mobility and/or aerodynamic diameter conversion without any knowledge about the particle&#39;s shape and its optical properties in the aerosol being characterized. In one example embodiment of the invention, the method includes generating a set of calibration data and finding the optimal refractive index and shape that best fits the calibration data. In addition, the method includes creating a new calibration curve that provides a mobility-equivalent or aerodynamic-equivalent diameter.

CLAIM OF PRIORITY AND CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional Patent Application ofnon-provisional patent application Ser. No. 14/236,372, with a 371(c)date of Mar. 31, 2014, claims priority to International Application No.PCT/US2012/049994, filed on Aug. 8, 2012, which in turn claims thebenefit of U.S. Provisional Application No. 61/521,614, filed Aug. 9,2011, and is related to U.S. Pat. No. 7,932,490 issued on Apr. 26, 2011,the disclosures of which are hereby incorporated by reference in theirentireties.

BACKGROUND OF THE INVENTION

Aerosols commonly found in the environment are generated both by natureand human activity. They influence human lives in many ways. Aerosols inthe atmosphere can absorb and/or scatter light and change visibility aswell as the earth energy balance. Atmospheric aerosols also serve ascondensation sites for cloud formation, thus playing an important rolein the climate. When inhaled, aerosol particles can deposit on therespiratory track and cause adverse health effects.

Industry and government have recognized the importance of measuring andmonitoring aerosol concentrations in the environment or workplace sothat proper measure can be taken to reduce potential health risks.Pertinent monitoring applications include but are not limited toindustrial/occupational hygiene surveys, outdoor ambient/site perimetermonitoring for dust control operations, and engine emission studies.Some industrial processes require knowledge of the particulates in theenvironment, including environments having a sparse population ofparticles (e.g., semiconductor manufacturing) as well as environmentshaving an extensive presence of particle populations (e.g., dry powdermanufacturing processes).

In 1987, the United States Environmental Protection Agency (EPA) revisedthe National Ambient Air Quality Standards (NAAQS) and started to usemass of particles with aerodynamic diameters less than approximately 10μm (hereinafter “the PM10”) as the particulate matter (PM) pollutionindex. The PM10 is an index of the PM that can enter the thorax andcause or exacerbate lower respiratory tract diseases, such as chronicbronchitis, asthma, pneumonia, lung cancer, and emphysema. It was laterdetermined that PM concentrations in the air, as indexed by the mass ofparticles with aerodynamic diameters less than approximately 2.5 μm(“PM2.5”) was more closely associated with the annual mortality ratesthan with the coarser PM10. In 1997, in its next revision of the NAAQS,the EPA promulgated regulations on PM2.5. Recently, there has beenextensive discussion on the health effects of particles smaller than 1μm (i.e. “PM1”).

The American Conference of Governmental Industrial Hygienists (ACGIH)has also established sampling conventions of respirable, thoracic andinhalable aerosols, defined as particles having aerodynamic diameters ofless than 4 μm, 10 μm, and 100 μm respectively. Inhalable particles arethose capable of entering through the human nose and/or mouth duringbreathing. Thoracic particles are the inhaled particles that maypenetrate to the lung below the larynx. Respirable particles are theinhaled particles that may penetrate to the alveolar region of the lung.A discussion of the various sampling conventions are found at NationalPrimary and Secondary Ambient Air Quality Standards, 40 Code of USFederal Regulation, Chapter 1, Part 50 (1997) and Vincent, J. H.,Particle Size-Selective Sampling for Particulate Air ContaminantsCincinnati, ACGIH (1999), both of which are hereby incorporated byreference except for explicit definitions contained therein.

While the aforementioned standards and conventions are based on theaerodynamic diameters of particles, it is understood that sizesegregated mass concentration groupings (e.g., PM1, PM2.5, PM10,respirable, thoracic and inhalable) may be based on the optical particlediameters instead of the aerodynamic diameters for purposes of theinstant application. That is, PM2.5 (for example) may approximateparticles having an aerodynamic diameter of less than approximately 2.5μm or particles having an optical diameter of less than approximately2.5-μm.

One instrument that measures particle size dependent numberconcentrations in real time is the optical particle counter (OPC). In anOPC, particles pass through an interrogation volume that is illuminatedby a light beam. The light scattered by each particle is collected on toa detector to generate an electrical pulse. From the pulse height (i.e.the intensity of the scattered radiation) one can infer the particlesize based on prior calibration. Because the size inferred from the OPCdepends on the particle optical properties, the inferred parameter isoften referred to as the “optical equivalent particle size.” By assumingaerosol properties such as density, shape and refractive index, the sizedistribution can be converted to mass distribution, such as described byBinnig, J., J. Meyer, et al. “Calibration of an optical particle counterto provide PM2.5 mass for well-defined particle materials,” Journal ofAerosol Science 38(3): 325-332 (2007), which is hereby incorporated byreference herein other than express definitions of terms specificallydefined therein. Some advantages of the OPC are: (1) particles may becounted with high accuracy for low particle concentrations; (2)favorable signal to noise ratios for particle sizes greater than 1 μm;and (3) low cost. However, the inferred particle optical size may not bethe same as the actual or geometric particle size because thedetermination depends on the particle shape and refractive indexassumptions.

Another instrument that measures particle size distribution in real timeis an Aerodynamic Particle Sizer (APS), such as described in U.S. Pat.No. 5,561,515 to Hairston et al., assigned to the assignee of theinstant application, the disclosure of which is hereby incorporated byreference herein other than express definitions of terms specificallydefined therein. When particles of different sizes are acceleratedthrough an accelerating nozzle, larger particles may tend to beaccelerated to a lesser extent through the interrogation volume(s) thansmaller particle because the larger particles may possess a greaterinertia to overcome. The APS exploits this principle by acceleratingparticles through a nozzle to obtain size dependent particle velocities,which are typically measured by measuring the time-of-flight of theparticles through the sensing zone. Unlike the OPC measurement, the APSmeasurement is independent of the particle refractive index. Also, whileconverting the particle size distribution to mass distribution, the APSis less sensitive to the particle density parameter than the OPCmeasurement. Good agreement between the mass concentrations calculatedfrom APS spectra and from direct mass measurements has been demonstratedin the size range of 0.5- to 10-μm. See Sioutas, C. (1999). “Evaluationof the Measurement Performance of the Scanning Mobility Particle Sizerand Aerodynamic Particle Sizer.” Aerosol Science and Technology 30(1):84-92.

A shortcoming of the APS is that only particle populations of relativelylow concentration (e.g., on the order of 1000-particle/cm³ and lower)can be measured due to coincidence error. For example, the TSI Model3321 APS accurately measures aerodynamic particle size distributions inthe 0.5- to 20-μm range, (with 5% coincidence error) up to approximately1000-particles/cm³. The APS resolution decreases with the particle size.Also, all commercially available instruments are relatively expensive.

The TSI Model 3321 APS utilizes the aerodynamic particle diameters ofthe detected particles to calculate the mass concentration of theaerosol. Effectively, the mass of each detected particle is calculatedassuming the particle to be spherical and of known density. Calibrationfactors may also be applied to account for correct the non-sphericalshape and differing density of the particles. Inherent limitations tothis approach are that the mass calculation is not based on detection ofthe smaller diameter particles (less than approximately 0.3-μm opticalor aerodynamic diameter) that go undetected by the APS or OPC detector.Also, this approach is limited to low concentration applications.

In spite of several shortcomings, there are numerious arguments in favorof using optical particle counters (OPCs) to measure particle sizedistribution and to perform mass concentration measurement. To make theOPCs more robust and address some of the shortcomings, there is a needto develop a method or measurement system to convert optical diametersto measures of diameter related to aerosol particle physical behavior,such as electrical mobility (or simply mobility) and/or aerodynamicdiameters. This conversion is advantageous because mobility diametersare more commonly used for submicron particles (particles with sizessmaller than 1 μm), while aerodynamic diameters are commonly used inareas such as aerobiology, heath effect studies, mass measurement, etc.This conversion can be done if the optical properties of the aerosols ofinterest are known, and the particles in aerosols are spherical.However, most of the aerosols of interest have particles of irregularshape and their optical properties are usually unknown as well, so theconversion cannot be easily made.

SUMMARY OF THE INVENTION

The optical particle counter (OPC) is one of the most widely usedaerosol instruments because of its low cost and ability to rapidlyprovide particle size distributions in real time. OPCs measure the sizeand number concentration of aerosol particles by means of lightscattering by single particles. As each particle passes through afocused light beam, it scatters a pulse of light to a photodetector,which is then converted to an electric signal. The focused light beamcould come from a white light or laser source. The electric signal isusually an electronic pulse. This pulse is analyzed and the pulse heightor area is then correlated to particle size and the count is distributedto the proper size channel, where the total counts in each size rangeare accumulated. In addition to the particle size, the amount of lightscattered by the particle also depends on the particle properties,namely refractive index and shape.

Particle diameters measured by OPCs are usually referred to as opticaldiameters. Since almost all commercially available OPCs arefactory-calibrated with polystyrene latex (PSL) particles, sometimes thediameters are also referred to as PSL-equivalent diameters.Nevertheless, in most of the applications, optical diameters orPSL-equivalent diameters usually are not very useful, and they need tobe converted to measures of diameter related to their physical behavior,such as electrical mobility (or simply mobility) and/or aerodynamicdiameters. This conversion is necessary because mobility diameter ismore commonly used for particles smaller than 1 μm, while aerodynamicdiameters are more commonly used in areas such as aerobiology, healtheffect studies, environmental monitoring, etc. The conversion can bedone if aerosol particles are spherical in shape and their refractiveindices are known since the light scattering and extinction by aspherical particle can be described and modeled by the Mie scatteringtheory. Unfortunately, except for certain laboratory generated aerosolparticles, aerosols of interest are usually irregular in shape and/ortheir refractive indices are unknown.

Size distributions of airborne particles often span a wide size rangefrom a few nanometers to several micrometers, which typically exceedsthe measurement size range of any single instrument. Therefore,researchers often combined data from multiple instruments. One suchcombination is that of electrical mobility-based instrument such as thescanning mobility particle sizer (SMPS) and light scattering-basedinstrument such as the optical particle counter (OPC). The SMPS isregarded as the gold standard for submicron aerosol size distributionmeasurement. Depending on the configuration, it can cover the size rangefrom 2.5 nm to 1 μm. The OPC is one of the most widely used instrumentsfor coarse particles especially in areas such as filter testing, indoorair quality, cleanroom monitoring, etc. Typical OPC size range is 0.3 to10 μm.

The SMPS and OPC have different measurement principles. The SMPSclassifies particles according to their electrical mobilities, and forspherical particles, the electrical mobility sizes are same as thegeometric sizes. The OPC, on the other hand, measures sizes according tothe amount of light scattered by the particles. The light scatteringphenomenon can be described by the Mie scattering theory. The sizesmeasured by the OPC are typically referred to as optical diameters.Because of the different measurement principles, in order to combineSMPS and OPC distributions into a single size spectrum, particle shapefactors and refractive indices are needed. Unfortunately, except forsome laboratory generated aerosols, shape factors and refractive indicesof aerosols of interest are typically unknown. In this study weattempted to fit the optical particle size distributions to the SMPSdistributions for aerosols with known and unknown refractive indices.For aerosols with known refractive indices, the optical sizedistributions were adjusted with Mie scattering calculation. Ifrefractive indices were unknown, an additional calibration step wasperformed. Several laboratory generated aerosols with known refractiveindices were used to evaluate the method. The method was then furtherevaluated with various ambient aerosols with unknown refractive indices.

The various embodiments of the inventive concept disclosed hereindisclose a system and a method that can measure a particle's size in aselect aerosol using the optical diameter of the particle to perform amobility and/or aerodynamic diameter conversion without any knowledgeabout the particle's shape in the aerosol being characterized and itsoptical properties. In one example embodiment of the invention, themethod includes generating a set of calibration data and finding theoptimal refractive index and shape that best fits the calibration data.In addition, the method includes creating a new calibration curve thatprovides a mobility-equivalent or aerodynamic-equivalent diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram depicting a particle size measurementsystem in an embodiment of the invention.

FIG. 1A is a depiction of the system and method for converting opticaldiameters into mobility and/or aerodynamic diameters.

FIG. 2 depicts the calibration procedure for converting opticaldiameters to mobility diameters.

FIG. 3 depicts the calibration procedure for converting opticaldiameters to aerodynamic diameters.

FIG. 4 depicts an example flow of a conversion from optical diameter toaerodynamic diameter.

FIG. 5 depicts particle distributions measured with an optical sizer anda mobility diameter sizer.

FIG. 6 depicts particle size distribution measured at a physicalfacility.

FIGS. 7A-7F are various flowcharts that illustrate various components ofthe calibration process and the optical to mobility/aerodynamic diameterconversion method.

FIG. 8 depicts a measurement system using a SMPS and OPS to measureparticle size over a wide range.

DETAILED DESCRIPTION OF THE INVENTION

Following are more detailed descriptions of various related conceptsrelated to, and embodiments of, methods and apparatus according to thepresent disclosure. It should be appreciated that various aspects of thesubject matter introduced above and discussed in greater detail belowmay be implemented in any of numerous ways, as the subject matter is notlimited to any particular manner of implementation. Examples of specificimplementations and applications are provided primarily for illustrativepurposes.

Referring to FIG. 1, a particle size measurement system 30 comprising anaerosol measurement section 32 and a calibration and processing section(CAPS) 34 is schematically depicted in an embodiment of the invention.An incoming flow stream 36 may be drawn from an aerosol cloud 38 throughan inlet 40 of the aerosol measurement section 32. The incoming flowstream 36 may be split into a sheath flow stream 42 and an aerosol flowstream 44 having particles that are targeted for measurement. The sheathflow stream 42 may be diverted to a sheath flow conditioning loop 45that may include a filtration device 46 and a flow measuring device 47.The aerosol flow stream 44 may be passed through an inlet nozzle 49 toan optics chamber 48 that includes a viewing or interrogation volume 50.The interrogation volume 50 may be defined by the intersection of alight beam 54 and the aerosol flow stream 44. The light beam 54 may besourced from an electromagnetic radiation source 56 such as a diodelaser, a LED or a lamp (broadband or line emitting). In a relatedembodiment (discussed later herein; see FIG. 8), the sheath flow comesfrom air that is exhausted from blower 98 which is then filtered andre-routed to loop 45 to act as sheath flow 42 before reaching nozzle 49.

In this example embodiment, particle size measurement system 30 includesbeam shaping optics 60 that may include a lens 62 such as a cylindricallens. The shaping optics 60 may additionally or alternatively comprisereflective components such as mirrors, or fiber optic components (notdepicted). A portion of the light scattered from particles over a solidangle 64 may be subtended by a light collection system or radiationcollector 66 (e.g., a spherical mirror, aspheric condenser lenses, orother electromagnetic radiation collection devices available to theartisan) within the optics chamber 48. An unscattered portion 70 of thelight beam 54 may be captured by a light trap 72. Inner surfaces 74 ofthe optics chamber 48 may be coated with a black or high absorptivitycoating such as an anodized coating. Collected light 78 gathered by theradiation collector 66 may be transferred to a detector 80 such as aphotodiode or a photomultiplier tube. The detector 80 may produce anelectrical signal 82 proportional to the convolution of the incidentelectromagnetic radiation and the spectral sensitivity of the detector80.

In some embodiments, the aerosol flow stream 44 exits the optics chamber48 through an outlet nozzle 84 and may be passed through a gravimetricfilter 86, thereby producing a pre-filtered aerosol flow stream 88. Theaerosol flow streams 44 may be drawn through the optics chamber 48 by apumping system 90 that includes a protection filter 92, a flowmeter 94,a flow pulsation damping chamber 96 and a pump or blower 98 that isducted to an exhaust 99. Numerous kinds of pumps or blowers may beutilized, including but not limited to a diaphragm pump, a rotary vanepump, a piston pump, a roots pump, a linear pump or a regenerativeblower.

In one embodiment, the CAPS 34 may condition the electrical signal 82 todefine three different signal circuits: a dynamic mobility diametersignal circuit 100 for generating mobility diameter data/outputs 102associated with the particles of the collected light 78 gathered by theradiation collector 66 and incident on the detector 80; a pulse heightconditioner circuit 104 for detecting scattered light originating fromindividual particles as they pass through the interrogation volume 50and generating pulse height outputs 106 in accordance therewith; and anaerodynamic diameter signal circuit 108 generating aerodynamic diameterdata/outputs 110 that provide (direct or indirect) measurement of thesize of the particles being measured as they pass through theinterrogation volume 50. The outputs 102, 106 and 110 may be routed to adigital processor module 114 for calibration and subsequent conversioninto a particle size distribution 113. The result can be output to adevice 116, such as a display, a storage device, analog output or acomputer.

Functionally, beam shaping optics 60 may be utilized to configure theshape of the light beam 54 and interrogation volume 50 to possesscertain characteristics, such as overall width and height, as well asintensity profile. The light trap 72 mitigates or prevents biasing ofthe electrical signal 82 that may be caused by the unscattered portion70 of the light beam 54 gathered by the radiation collector 66 aftermultiple scattering within the optics chamber 48. When utilized, thehigh absorptivity coating on the inner surfaces 74 of the optics chamber48 may further reduce the propagation of stray light. In operation,mobility diameter circuit 100 provides data akin to that produced by adifferential mobility analyzer device while pulse height output 106 andaerodynamic diameter data/output 110 are akin to the outputs of OPC andAPS devices, respectively.

Particles can be collected on the gravimetric filter 86 and can beweighed to measure mass directly. This direct mass measurement can beused to create the calibration relationship between the electricalsignal 82 and the mass of the collected particles (see discussionattendant FIG. 9 of U.S. Pat. No. 7,932,490 and the Binnig 2007 paperreferenced herein). Particles on the gravimetric filter 86 can also beanalyzed to study their chemical compositions. The protection filter 92may remove particles remaining in the air stream 88 upstream of theflowmeter 94 and pump or blower 98 for protection against particlecontamination, especially in configurations where there is nogravimetric filter in place. The pump or blower 98 may be used to drivethe flow through the whole system. The flow pulsation damping chamber 96is an optional device that may reduce the pulsation of flow in thesystem.

The filtration device 46 of the sheath flow conditioning loop 45 removesparticulate matters from the sheath flow stream 42 to provide asubstantially clean flow of gas that shrouds or sheaths the aerosol flow44. The cleansed sheath flow 42 helps contain particulates within thecore of the aerosol flow 44 as it passes through the optics chamber 48,thereby mitigating against particulate contamination of the opticschamber 48 and appurtenances therein. The flow measuring device 47, whenutilized, can provide an indication of the flow rate of the sheath flowstream 42 which can be subtracted from the total flow rate of theincoming flow stream 36 provided by the flowmeter 94 to determine theflow rate of the aerosol flow stream 44.

The optical particle counter (OPC) is one of the most widely usedaerosol instruments because of its low cost and ability to rapidlyprovide particle size distributions in real time. OPCs measure the sizeand number concentration of aerosol particles by means of lightscattering by single particles. In addition to the particle size, theamount of light scattered by the particle also depends on the particleproperties, namely refractive index and shape.

Particle diameters measured by OPCs are usually referred to as opticaldiameters or PSL-equivalent diameters. Nevertheless, in most of theapplications, optical diameters usually are not very useful, and theyneed to be converted to measures of diameter related to their physicalbehavior, such as electrical mobility and/or aerodynamic diameters. Thisconversion is necessary because mobility diameter is more commonly usedfor particles smaller than 1 μm, while aerodynamic diameters are morecommonly used in areas such as aerobiology, health effect studies,environmental monitoring, etc. The conversion can be done if aerosolparticles are spherical in shape and their refractive indices are knownsince the light scattering and extinction by a spherical particle can bedescribed and modeled by the Mie scattering theory. Unfortunately,except for certain laboratory generated aerosol particles, aerosols ofinterest are usually irregular in shape and/or their refractive indicesare unknown. The following description provides more detail on theoverall measurement system and on the operation of CAPS module 34 whicheventually helps to generate more accurate and robust particle sizemeasurement data by using calibration data generated along with Miescattering modeling.

Referring now to FIG. 1A, in one example embodiment of a measurementsystem 100A for particles in an aerosol to be measured with an opticalparticle sizer (OPS) 110A, which uses therein an OPC, that isoperatively coupled to a calibration system 120A and a refractive indexand a shape factor system 130A. Together, OPS 110A, calibration system120A and system 130A optimize the refractive index and the shape factorin 140A thereby generating mobility and/or aerodynamic size calibratedOPS 150A. In general, the method of the present invention consists ofthree main steps: (1) generating a set of calibration data; (2) findingthe optimal refractive index and shape that best fits the calibrationdata; and (3) creating new calibration curves that providemobility-equivalent or aerodynamic-equivalent diameters. In this figure,the OPS is represented by a TSI 3330 Optical Particle Sizer (whichincludes an OPC).

Referring now to FIGS. 2-4, the calibration steps for mobility andaerodynamic diameters are different as will be discussed herein. Forgenerating calibration data for the mobility diameter application, acalibration system 200 uses an OPC 210 and a differential mobilityanalyzer (DMA) 220 and involves generating several differential mobilityanalyzer classified monodisperse aerosols and subsequently measuringthem with the OPC. Next, system 200 optimizes the refractive index tobest fit the monodisperse data at step 230.

Referring now to FIG. 3, in generating calibration data for anaerodynamic diameter application (which can include a number ofdifferent ways to generate aerodynamic data depending on the set up andcomponents used), for the sake of simplicity, we focus on system 300 andon data generated with impactors or cyclones 320. The size distributionof the particles in the aerosol of interest is measured by an OPC 310with and without the impactor or a cyclone. At step 340, a cut point ofimpactor or cyclone 320 is then determined by taking the ratio of thetwo OPC distributions generated on the previous step. Since the OPCresponse is based on a factor-calibrated PSL curve (350), the cut pointmeasured by the OPC is not expected to be the same as theimpactor/cyclone cut point which is defined using the aerodynamicdiameter. By using several different cut point impactors or cyclones, aset of calibration data can be obtained. Once the calibration data isdetermined, a Mie scattering (modeling) calculation is then performed tofind the refractive index that best fits the calibration data. In arelated embodiment, a shape factor is also included in the calculationto improve the accuracy. In this example embodiment, the refractiveindex and shape factor that provide the best fit to the calibration dataare referred to as optimal refractive index and optimal shape factor. Anew mobility-equivalent or aerodynamic-equivalent calibration curve isthen created using this optimal refractive index and shape factor.

Referring now to FIG. 4, in this example embodiment OPS 410 of system400 is calibrated with impactors 420 with cut points 1, 2.5 and 10 μmusing the calibration steps described in FIG. 3. Since the OPS measuresoptical or PSL-equivalent diameters, the cut points measured aredifferent from the impactor cut points which are based on aerodynamicdiameters. In this example embodiment, OPS 410 responses for impactors420 are 1.5, 4 and 12 μm. A Mie scattering calculation program 430 isthen used to find the optimal refractive index and shape factor so thatcut points measured with the OPS are as close as possible to the knownimpactor cut points of 1, 2.5 and 10 μm. As shown in FIG. 4, OPS 410measured cut points after refractive index and shape factor correction(RIC) are found to be 1.1, 2.6, and 10.2 μm at step 440. Ideally, wewould like these values exactly the same as the impactor cut points 1,2.5 and 10 μm. In practice, however, accuracy is limited by instrumentresolution, the number of calibration point as well as particleproperties. Once the optimal refractive index and shape factor aredetermined, a new calibration curve is generated and the OPS can now beused to measure aerodynamic diameter equivalent diameter for thisaerosol.

One application of a mobility-diameter-calibrated OPC of system 200 isthat it can be combined with a Scanning Mobility Particle Sizer (SMPS)for wide range particle size distribution measurement (see FIG. 8).Since the typical SMPS size range is from a few nanometers (nm) to about500 nm, and an OPC size range is from 300 nm to 10 μm, combining thesetwo instruments would allow size distribution measurement from a fewnanometers to about 10 μm. FIG. 5 shows an example of particle sizedistributions (Sample #13) measured with an SMPS and OPS. It is clearthat the OPS distribution after the RIC merges better than the onewithout the RIC to generate a more accurate and consistent curve ofmeasurement of the wide particle size range. FIG. 6 shows particle sizedistributions measured in a designated smoking area (Sample #5), with asecond peak (around 150 nm) shown in the figure to be particlesgenerated from cigarettes.

Referring now to FIGS. 7A-7F, there are illustrated various flowchartsthat describe various components of the calibration process and theoptical to mobility/aerodynamic diameter conversion method. Inparticular, FIG. 7A is the top level flowchart showing the steps tomodifying an optical particle sizer and/or an optical detector to arriveat a mobility diameter-ready OPC device 7800 or an aerodynamicdiameter-ready OPC device 7900. In this example embodiment, the device7800 will be used as described in box 7850 to measure particles fromabout 10 nanometers (nm) to about 10 micrometers (μm), which is tocombine an SMPS and resized OPC distributions.

Referring more specifically to FIG. 7A, a method of recalibrating an OPCincludes the step 7100 of measuring polydisperse aerosol sizedistributions with an OPC, after which the system requests at step 7200if the OPC is ready for calibration. If the OPC is not ready, the userdetermines how often to do the calibration depending, among otherfactors, if there are, for instance, rapidly changing aerosols (whichmay require more calibrations). If the OPC is ready for calibration,then proceed to a calibration procedure 7300, which can be thecalibration procedure 7300A (FIG. 7B) for mobility-diameter ready OPC7800 or for procedure 7300B (FIG. 7C) for aerodynamic diameter-ready OPC7900.

Referring again to FIG. 7A, at step 7400 the user determines if there isenough calibration data, depending on the desired accuracy of resultsand the time available (the higher the accuracy desired, the longer ittakes due to need for more calibration data). Once calibration foreither one (7300A or 7300B) is complete at step 7400, the next step 7500is to find the optimal refractive index and shape factor using thecalibration data set. Step 7500 can then be subdivided into 3 subflows:Flow 7500A (FIG. 7D) for determining optimal refractive index and shapefactor for mobility diameter using one calibration data; Flow 7500B(FIG. 7E) for determining optimal refractive Index and shape factor formobility using multiple calibration data; and Flow 7500C (FIG. 7F) fordetermining optimal refractive index and shape factor for aerodynamicdiameter using multiple calibration data.

Referring back to FIG. 7A, once the optimal refractive index and shapefactors are found, the next step 7600 is to generate a new calibrationcurve using the acquired optimal refractive index and shape factor. Atstep 7700, a resizing operation of the polydisperse aerosols measured inthe first step with the new calibration curve is then performed. At thispoint, depending on the original calibration, the process bifricates tostep 7800 to be a mobility diameter-ready OPC or to step 7900 to be anaerodynamic diameter-ready OPC.

Referring now more specifically to FIG. 7B and mobility calibrationprocess 7300A for the mobility-ready OPC 7800, a particle and an aerosolof interest (polydisperse) is introduced to the DMA thereby generatingparticle mobility data for one particular size particle which is theninput into the OPC to generate a response. If there is enoughcalibration data, then the calibration data for each particle along withits optical diameter is generated. If more calibration data is needed,the DMA voltage is changed to generate another single size particlewhich is then input into the OPC to generate another optical response.

Referring now more specifically to FIG. 7C for procedure 7300B foraerodynamic diameter-ready OPC 7900, an aerosol of interest(polydisperse) is introduced first to the OPC to generate opticaldiameters of the particles of interest. The particle flow is alsointroduced to the impactor/cyclone to generate aerodynamic diameterswith its flow than being introduced to the OPC to then generate opticaldiameters of the particles of interest. If there is enough calibrationdata, then the calibration data for each particle along with its opticaldiameter is generated for each of its aerodynamic diameter (with andwithout the impactor/cyclone data). If more calibration data is needed,the cut point for the impactor/cyclone is changed and then the particleis measured again in the impactor/cyclone path and moves back through tothe OPC to then determine if there is enough calibration data.

Referring now more specifically to FIG. 7D (and step 7500 of FIG. 7A),subflow 7500A illustrates how to determine the optimal refractive indexand shape factor for mobility diameter using one calibration data byfirst introducing the aerosol of interest (polydisperse) to the DMAthereby generating particle mobility data for one particular sizeparticle which is then input into the OPC to generate a response. If theparticle optical diameter is equal to the mobility diameter then theoptimal refractive index has been found which leads to generating a newOPC calibration curve 7600A and then a mobility diameter calibrated OPC7800A. On the other hand, if the particle optical diameter is not equalto the mobility diameter then the refractive index is adjusted,thereafter performing a Mie scattering calculation which then generatesa new OPC response or optical diameter. This keeps cycling until the theparticle optical diameter is equal to the mobility diameter then theoptimal refractive index has been found and then it moves to therighthand of flow 7500A leading to creating a new OPC calibration curve7600A as discussed above.

Referring now more specifically to FIG. 7E (and step 7500 of FIG. 7A),subflow 7500B illustrates how to determine the optimal refractive indexand shape factor for mobility using multiple calibration data. Theaerosol of interest (polydisperse) is introduced to the DMA therebygenerating particle mobility data for one particular size particle whichis then input into the OPC to generate a response. If there is enoughcalibration data, then the calibration data for each particle along withits optical diameter is generated. If more calibration data is needed,the DMA voltage is change and then the particle is measured again in theDMA and back through to the OPC. Once the calibration data generatedthen the chi-square is calculated and its determined if we have reachedthe minimum chi-square, if so then the optimal refractive index has beenfound which leads to generating a new OPC calibration curve 7600B andthen a mobility diameter calibrated OPC 7800B. On the other hand, if theminimum chi-square is not found then the refractive index is adjusted,thereafter performing a Mie scattering calculation. This in turngenerates a new OPC response or optical diameter and thereafter anotherchi-square is calculated and compared with a minimum chi-square. Thiskeeps cycling until the the minimum chi-square is reached then theoptimal refractive index has been found and then it moves to therighthand of flow 7500B leading to creating a new OPC calibration curve7600B as discussed above.

Referring now more specifically to FIG. 7F (and step 7500 of FIG. 7A),subflow 7500C illustrates how to determine the optimal refractive indexand shape factor for aerodynamic diameter using multiple calibrationdata. The aerosol of interest (polydisperse) is introduced first to theOPC to generate optical diameters of the aerosol particles of interest.The particle flow is also introduced to the impactor/cyclone to generateaerodynamic diameters with its flow than being introduced to the OPC tothen generate optical diameters of the particles of interest. If thereis enough calibration data, then the calibration data for each particlealong with its optical diameter is generated for each of its aerodynamicdiameter (with and without the impactor/cyclone data). If morecalibration data is needed, the cut point for the impactor/cyclone ischanged and then the particle is measured again in the impactor/cyclonepath and moves back through to the OPC to then determine if there isenough calibration data. Once the calibration data is generated then thechi-square is calculated and its determined if we have reached theminimum chi-square, if so then the optimal refractive index has beenfound which leads to generating a new OPC calibration curve and then anaerodynamic diameter calibrated OPC 7900C. On the other hand, if theminimum chi-square is not found then the refractive index is adjusted,thereafter performing a Mie scattering calculation. This in turngenerates a new OPC response or optical diameter and thereafter anotherchi-square is calculated and compared with a minimum chi-square. Thiskeeps cycling until the the minimum chi-square is reached then theoptimal refractive index has been found and then it moves to therighthand of flow 7500C leading to creating a new OPC calibration curveas discussed above.

In a related embodiment, a DMA and OPS particle sizing system includes aswitch to allow for re-calibration by having the particle flow only tothe DMA and then once its re-calibrated then the system switches backover to having the particle flow through both the DMA and the OPS.

Referring now to FIG. 8, in this example embodiment of the measurementsystem, 8000, an SMPS system used in this work was a TSI Model 3936 witha differential mobility analyzer (DMA) and condensation particle counter(CPC) being an LDMA Model 3081 and CPC Model 3010, respectively. Thesize range was 10 nm to about 500 nm. The OPC used in this work was ahigh resolution optical particle spectrometer TSI 3330 Optical ParticleSizer (OPS). The OPS is a light portable, battery-powered unit that iscapable of detecting particles from 0.3 to 10 μm in diameter in up to 16channels. The channel boundaries are user adjustable. The OPS alsofeatures real-time Mie scattering calculation capability.

In this example embodiment of the measurement method, a polydisperseaerosol was first measured by the SMPS and OPS simultaneously. Tocalibrate the OPS for mobility diameter, several DMA classifiedmonodisperse aerosols were measured by the OPS. To improve data quality,the size channel boundaries of the OPS were adjusted so that all 16channels were allocated over the narrow range from 0.3 to 1.0 μm forhigh resolution measurement. Mie scattering calculations were thenperformed to find the optimal refractive index that minimized thedifference between the mobility diameters of these monodisperse aerosolsfrom the DMA and optical diameters from the OPS. The shape factor wasalso used to further improve the results. This calibration step isillustrated in FIG. 2. Once the optimal refractive index and shapefactor were determined, a new calibration curve was generated, and OPSpolydisperse aerosol distributions measured in the first step were thenresized with this new calibration curve. The resized OPS polydispersedistributions were subsequently merged with the SMPS distributions toobtain wide range aerosol size distributions. Challenge aerosols:Methylene blue, Dioctyl sebacate (DOS or DEHS), and ambient aerosolsmeasured at various locations.

Distributions of a laboratory generated methylene blue aerosol measuredwith SMPS and OPS were previously shown in FIGS. 5 and 6. Agreementbetween the SMPS distribution and OPS distribution with the refractiveindex correction in the overlapping region is clearly better than theOPS distribution without the correction. Ambient aerosol sizedistributions measured with the SMPS and OPS at one location areillustrated in FIG. 5, while another location, designated as a smokingarea, appears as the second peak in FIG. 6 and is believed to beparticles generated from cigarettes.

A method was successfully developed to convert optical diameters tomobility diameters without the knowledge of aerosol shape and opticalproperties. Without the refractive index adjustment and shape factorcorrection, it was found that optical diameters could be quite differentfrom the mobility diameters if the refractive indices of the aerosolswere very different from the PSL aerosols (such as methlyne blueaerosol) and/or aerosols were nonspherical. The disclosed method is morerobust than merging SMPS and OPS distributions by minimizing the countdifferences between the two measurement techniques, since the mergeddistributions by the latter method could be significantly biased byinstrument counting efficiencies in the overlapping region.

In one example embodiment, a measurement system for measuring aerosolsize distribution includes an electromagnetic radiation sourceoperatively coupled with beam shaping optics for generation of a beam ofelectromagnetic radiation; an inlet nozzle for passage of an aerosolflow stream therethrough, said aerosol flow stream containing particlesand intersecting said beam of electromagnetic radiation to define aninterrogation volume, said particles scattering said electromagneticradiation from said interrogation volume; and a radiation collector forcollection of a portion of said electromagnetic radiation scattered fromthe particles in said interrogation volume. A detector is also includedfor detection of said portion of said electromagnetic radiationcollected by said radiation collector, along with a calibration systemfor generating one or more sets of calibration data from a particle ofan aerosol of interest, said calibration system operatively coupled tosaid detector. The system also includes digital processing means forcomputing an aerodynamic diameter and/or mobility diameter from anoptical diameter operatively coupled to said calibration system, saiddigital processing means configured to generate a Mie light scatteringmodel to determine a refractive index of the particles of an aerosol ofinterest using the calibration data and then convert the opticaldiameters to aerodynamic diameters and/or mobility diameters using theresulting refractive index.

In a related embodiment, the measurement system has a calibration systemthat is an electrical mobility device adapted to generate predefinedelectrical mobility size distributions or electrical mobility cutpoints. The electrical mobility device is selected from the groupconsisting of a differential mobility analyzer and electrostaticprecipitator.

In another related embodiment, the measurement system further includes ascanning mobility particle sizing device operatively coupled to thedetector thereby providing a wide particle range sizing system adaptedto measure a mass of particles having a lower limit defined as beingbetween about 10 nm and about 500 nm and an upper limit defined as beingbetween about 300 nm and about 10 μm.

In yet another related embodiment, the measurement system has acalibration system that is an aerodynamic diameter device adapted togenerate aerodynamic diameter cut points. The aerodynamic diameterdevice is an impactor or a set of impactors with different aerodynamiccut points. In a related embodiment, the aerodynamic diameter device isa cyclone or a set of cyclones with different aerodynamic cut points.

In another example embodiment, an instrument for measuring aerosol sizedistribution includes an electromagnetic radiation source operativelycoupled with beam shaping optics for generation of a beam ofelectromagnetic radiation; an inlet nozzle for passage of an aerosolflow stream there through, said aerosol flow stream containing particlesand intersecting said beam of electromagnetic radiation to define aninterrogation volume, said particles scattering said electromagneticradiation from said interrogation volume; and a radiation collector forcollection of a portion of said electromagnetic radiation scattered fromsaid interrogation volume. The instrument also includes a detector fordetection of said portion of said electromagnetic radiation collected bysaid radiation collector; and digital processing means for computing aMie light scattering model and adapted to incorporate optical propertiesof the particles in an aerosol of interest as part of the particlemeasurement, said digital processing means operatively coupled to saiddetector. In a related embodiment, digital processing means is alsoconfigured to convert optical diameters of the particles to aerodynamicdiameters and/or electrical mobility diameters, said digital processingmeans operatively coupled to said detector.

In another example embodiment, a method for determining electricalmobility aerosol size distribution includes providing a detector toreceive electromagnetic radiation scattered from an interrogation volumeand causing particles to flow through said interrogation volume andscatter electromagnetic radiation onto said detector to generate anelectrical signal from said detector. The method also includesgenerating a plurality of pulse height outputs from said electricalsignal with said pulse height signal conditioner, each of said pulseheight outputs corresponding to a particle passing through saidinterrogation volume and corresponding to an optical particle size, andincludes generating one or more sets of calibration data with acalibration system from the passing particles. The method furtherincludes the step of determining a refractive index of the particle ofan aerosol of interest using the calibration data and a Mie lightscattering model, and converting optical diameters of said passingparticles to electrical mobility diameters.

In one related embodiment, the method includes providing as thecalibration system an electrical mobility device which is capable ofgenerating narrow electrical mobility size distributions or providingknown electrical mobility cut points, wherein the electrical mobilitydevice is selected from the group consisting a differential mobilityanalyzer and an electrostatic precipitator.

In yet another example embodiment, a method for determining aerodynamicaerosol size distribution includes providing a detector to receiveelectromagnetic radiation scattered from an interrogation volume, andcausing particles to flow through said interrogation volume and scatterelectromagnetic radiation onto said detector to generate an electricalsignal from said detector. The method also includes generating aplurality of pulse height outputs from said electrical signal with saidpulse height signal conditioner, each of said pulse height outputscorresponding to a particle passing through said interrogation volumeand corresponding to an optical particle size, and generating one ormore sets of calibration data with a calibration system from the passingparticles. The method further includes determining a refractive index ofthe particles of an aerosol of interest using the calibration data and aMie light scattering model, and converting optical diameters of saidpassing particles to aerodynamic diameters. In a related embodiment, thecalibration system is an aerodynamic diameter device adapted to generateat least one predefined aerodynamic diameter cut point, wherein theaerodynamic diameter device is an impactor or a set of impactors withdifferent cut points. In another embodiment, the aerodynamic diameterdevice is a cyclone or a set of cyclones with different cut points.

In yet another example embodiment, a method for determining sizesegregated aerosol mass concentration includes providing a detector toreceive electromagnetic radiation scattered from an interrogationvolume, and causing particles to flow through said interrogation volumeand scatter electromagnetic radiation onto said detector to generate anelectrical signal from said detector. The method also includesgenerating a plurality of pulse height outputs from said electricalsignal with said pulse height signal conditioner, each of said pulseheight outputs corresponding to a particle passing through saidinterrogation volume and corresponding to an optical particle size andgenerating one or more sets of calibration data with a calibrationsystem. The method further includes determining the optimal/effectiverefractive index of the aerosol of interest using the calibration dataand the Mie light scattering model, converting optical diameters toaerodynamic diameters, and calculating a size segregated massconcentration from said aerodynamic diameters.

In a related embodiment, the calibration system of the method is anaerodynamic device which generates one or more known aerodynamicdiameter cut points, wherein the aerodynamic device is an impactor or aset of impactors with different aerodynamic cut points. In anotherembodiment, the aerodynamic device is a cyclone or a set of cycloneswith different aerodynamic cut points.

The following patents that relate to OPC devices are herein incorporatedby reference in their entirety and constitute part of the disclosureherein: U.S. Pat. Nos. 6,831,279; 5,561,515; 5,895,922; 6,639,671;7,066,037; and 7,167,099 and 7,932,490. Having thus described severalillustrative embodiments, it is to be appreciated that variousalterations, modifications, and improvements will readily occur to thoseskilled in the art. Such alterations, modifications, and improvementsare intended to be part of this disclosure, and are intended to bewithin the spirit and scope of this disclosure. While some examplespresented herein involve specific combinations of functions orstructural elements, it should be understood that those functions andelements may be combined in other ways according to the presentinvention to accomplish the same or different objectives. In particular,acts, elements, and features discussed in connection with one embodimentare not intended to be excluded from similar or other roles in otherembodiments. Accordingly, the foregoing description and attacheddrawings are by way of example only, and are not intended to belimiting.

I claim:
 1. A method for determining aerodynamic diameter aerosol sizedistribution of a mass of particles in an aerosol, comprising: providinga detector to receive electromagnetic radiation scattered from aninterrogation volume; causing the mass of particles to flow through saidinterrogation volume and scatter electromagnetic radiation onto saiddetector to generate an electrical signal from said detector; generatinga plurality of pulse height outputs from said electrical signal with apulse height signal conditioner, each of said pulse height outputscorresponding to a particle passing through said interrogation volumeand then correlating each pulse height output to an optical particlesize or diameter; generating one or more sets of calibration data with acalibration system from the passing mass of particles, wherein thecalibration system includes an aerodynamic diameter device adapted togenerate at least one predefined aerodynamic diameter cut point for anoptical particle diameter as part of generating the calibration data;determining a refractive index of the particles in the aerosol using thecalibration data and a Mie light scattering model calculation thatprovides the best fit to the calibration data then creating calibrationcurves that provide aerodynamic-equivalent diameters for particles usingthe refractive index, wherein the refractive index of the mass ofparticles is unknown before conducting a measurement on the mass ofparticles in the aerosol; and converting optical diameters of saidpassing particles to aerodynamic diameters using the refractive index.2. The method of claim 1 wherein the aerodynamic diameter device is animpactor or a set of impactors with different cut points.
 3. The methodof claim 1 wherein the aerodynamic diameter device is a cyclone or a setof cyclones with different cut points.
 4. A method for determining sizesegregated aerosol mass concentration of a mass of particles in anaerosol, comprising: providing a detector to receive electromagneticradiation scattered from an interrogation volume; causing the mass ofparticles to flow through said interrogation volume and scatterelectromagnetic radiation onto said detector to generate an electricalsignal from said detector; generating a plurality of pulse heightoutputs from said electrical signal with said pulse height signalconditioner, each of said pulse height outputs corresponding to aparticle passing through said interrogation volume and then correlatingeach pulse height output to an optical particle size or diameter;generating one or more sets of calibration data with a calibrationsystem from the passing mass of particles, wherein the calibrationsystem includes an aerodynamic device which generates one or more knownaerodynamic diameter cut points as part of generating the calibrationdata, then calculating a chi-square variable from the calibration dataand determining if a minimum chi-square variable defined value has beenreached; determining an optimal/effective refractive index of theaerosol of interest using the calibration data and a Mie lightscattering model calculation that provides the best fit to thecalibration data then creating calibration curves that provideaerodynamic-equivalent diameters using the optimal refractive index,wherein the refractive index of the mass of particles is unknown beforeconducting a measurement on the mass of particles in the aerosol;converting optical diameters of said passing particles to aerodynamicdiameters using the optimal/effective refractive index; and calculatinga size segregated mass concentration from said aerodynamic diameters. 5.The method of claim 4 wherein the aerodynamic device is an impactor or aset of impactors with different aerodynamic cut points.
 6. The method ofclaim 4 wherein the aerodynamic device is a cyclone or a set of cycloneswith different aerodynamic cut points.
 7. The method of claim 1, whereina new mobility-equivalent or aerodynamic-equivalent calibration curve isthen created using the refractive index and a shape factor, and whereinthe shape factor of the mass of particles is unknown before conducting ameasurement on the mass of particles in the aerosol.
 8. The method ofclaim 4, wherein a shape factor of the mass of particles is unknownbefore conducting a measurement on the mass of particles in the aerosol.9. The method of claim 4, wherein if the minimum chi-square variablevalue is not reached, then adjusting the refractive index and thenperforming a Mie scattering calculation which generates new opticaldiameters, thereafter calculating another chi-square and comparingminimum chi-square variable values.